DESCRIPTION: Signaling between epithelial cells and between the epithelium and mesenchyme is required for tooth morphogenesis and for continuous growth of postnatal rodent incisor teeth, a model system for studying Dental epithelial stem cells. Identification of the responsible intercellular signaling molecules and their targets is therefore essential for understanding inherited disorders of tooth development, and may ultimately reveal novel therapeutic strategies for treatment of tooth decay and for regenerating teeth in adults. Inhibition of paracrine WNT/beta-catenin signaling causes an early arrest of tooth development; conversely activation of the canonical WNT/beta-catenin pathway causes odontomas and supernumerary teeth in Gardner s syndrome patients, formation of ectopic tooth bud-like structures in transgenic mice, and loss of ameloblasts from postnatal mouse incisor teeth, suggesting that WNT signaling plays multiple roles in tooth development. The Eda and Pitx2 genes are candidate direct targets of WNT/beta-catenin signaling that are expressed in Dental epithelium and required for tooth development. We posit that specific canonical WNT proteins promote tooth fate in the embryo and regulate the proliferation of embryonic Dental primordia and ameloblast precursor cells in postnatal incisor teeth, and that these functions are mediated in part through activation of Eda and Pitx2. To test this hypothesis we propose three Specific Aims. In Specific Aim 1 we will identify cell populations that respond to WNT/beta-catenin signals during embryonic tooth development and in postnatal incisor teeth. In Specific Aim 2 we will dissect the relationship between Eda, Pitx2 and WNT in tooth development by: (i) determining whether Eda and Pitx2 require WNT/beta-catenin signaling for their expression at different stages of tooth development; (ii) testing whether forced expression of Eda or the Pitx2 target gene cyclin D2 can partially rescue the effects of WNT inhibition; and (iii) determining whether Eda or Pitx2 function is required for WNT induced promotion of tooth development. In Specific Aim 3 we will identify individual WNT proteins that are capable of promoting tooth development, and will determine whether activation of WNT/beta-catenin signaling is sufficient to induce expression of Eda and Pitx2, initiate tooth development, induce odontogenic tumors, or regulate the activity of incisor tooth ameloblast precursors. These experiments will help to place WNT signals in the network of regulatory factors that control tooth development, and will test the potential use of WNT activation in strategies for tooth or enamel regeneration.